In order to meet future emission regulations in vehicles having a compression ignition engine, it may be necessary to use lean NOx catalysts in combination with particulate filters.
A NOx catalyst reduces NOx emissions continuously, even in an oxygen rich environment. For an active NOx catalyst to maximize NOx reduction, a reducing agent, for example, diesel fuel or urea, needs to be present. The optimum amount of reducing agent for the NOx catalyst is typically based on engine operating conditions and catalyst conditions. These conditions typically include engine speed, engine load, and catalyst temperature.
A particulate filter, also commonly used with compression ignition engines, is used to prevent soot, or carbon particles, from exiting the tailpipe. Since the particulate filter has a limited storage capacity, it is periodically regenerated. In one approach, during the regeneration process, exhaust temperature is increased to ignite carbon particles stored in the particulate filter. By burning the stored carbon particles, the filter is regenerated and able to again store the carbon particles. In addition, the burning of the carbon particles causes an increase in temperature.
The inventor herein has recognized a disadvantage when combining the above systems, for example, by placing the NOx catalyst downstream of the particulate filter. In this configuration, when the particulate filter is being regenerated, excess reductants are made available to the NOx catalyst since reductants are released from the particulate filter during the regeneration process. Thus, reductant is fed to the NOx catalyst in excess of that needed to reduce NOx. This can result in increased HC and CO emissions.